1. Field of the invention
The invention concerns the connection of analog and digital subscriber lines, whether they are located near the digital local exchange or remotely therefrom, and is more particularly concerned with a digital telephone network subscriber exchange.
2. Description of the prior art
A digital local exchange must implement analog-to-digital conversion of speech signals from analog subscriber lines. This conversion is carried out in a subscriber line connection unit located either in the exchange itself or in the satellite exchange nearest the subscriber.
French Pat. No. 2 144 109 entitled Subscriber Line to Exchange Connection Unit for Time-division Switching Systems, corresponding to U.S. Pat. No. 3,941,947 describes a connection unit which may be either local, that is to say installed in the telephone exchange, or remote, that is to say constituting a satellite exchange connected to the exchange by 2 Mbit/s multiplex lines.
In digital exchanges, the concentration of subscriber lines is carried out either before or after analog-to-digital conversion, and the interfaces are always the same with analog subscriber lines on one side and 2 Mbit/s digital links on the other side.
Connection units of this kind with purely analog subscriber interfaces mean that a certain number of subscribers are without access to the advantages of digitization of the telephone network. Groups of geographically dispersed subscribers, generally remotely located, are connected to the exchange by means of shared lines, analog concentrators or analog multiplexers. All these devices are independent of the switching system and their installation and maintenance are handled by different personnel from the personnel responsible for the switching system.
Moreover, the current trend is to install integrated services digital networks which cover the full range of communication requirements: voice, data, images and sound. The integration of services must therefore begin in the distribution network with the provision at the subscriber's premises of a terminal installation controlling a set of terminals such as, for example, a telephone, a facsimile machine, a data terminal equipment, etc, and dialoging with the digital subscriber exchange over a digital line, operating at 144 kbit/s, for example, carrying voice, data and signaling simultaneously, coding being carried out on the subscriber's premises.
An object of the invention is to connect local and remote analog and digital subscriber lines in a way which offers all subscribers the advantages of a digital network.
Another object of the invention, in the case of subscribers located in an area remote from a digital local exchange, is to combine these subscribers by means of connections to a satellite unit located in their area and to integrate the satellite unit into the exchange in such a way that these subscribers are no longer in any way "special", either with regard to the services offered to them or with regard to operation and maintenance procedures.